The present invention relates to a reinforcement ply or fabric for elastomeric articles such as hoses, belts, pneumatic actuators, diaphragms or springs, conveyors and tires or any product which similarly relies on continuous reinforcing members for its structural performance. The present invention is particularly applicable to the structure and manufacture of breaker and bandage plies which reinforce the ground contacting tread of a tire.
The reinforcing plies commonly used in elastomeric products comprise a plurality of reinforcing members in the form of fibers, filaments or cords livid parallel to each other along the length of the ply and embedded in an elastomeric material such as a rubber compound. In the ply fabric all of the reinforcing members are identical to each other and the use of grooved spacing rollers and an accurately controlled calender ensures that they are equally spaced within a precise thickness of elastomeric matrix. Thus the ply is produced having uniform construction and reinforcing ability across its entire axial width.
Elastomeric articles in general, and tires in particular, may suffer from problems associated with changes of shape occurring either in the process of manufacture and/or in subsequent use. These changes in shape can lead to differential strains in the reinforcing plies incorporated therein which can have a detrimental effect on the structural integrity and performance of the product.
Tires, for example, are subject to changes in shape during manufacture and subsequently in service. Firstly in the assembly of the `green` or uncured tire, the reinforced carcass structure is changed from a cylinder to a toroid. Two subsequent changes which affect the tread reinforcing breaker occur first when the green tire is expanded into the mold prior to vulcanization and second when the finished tire is fitted to the wheelrim and inflated to its working pressure. Both of these situations change the shape of the initially cylindrical assembly of plies which form the breaker Accordingly differential strains are introduced into the breaker.
Further changes to the tire breaker occur under high speed running when the breaker belt edges tend to lift under the influence of centrifugal force. These further differential strains lead to a breakdown of the bond between the steel cords and the rubber matrix leading to so-called `breaker-edge looseness` which is a common cause of premature tire failure. Many high speed tire constructions attempt to mitigate this belt edge lifting problem by wrapping the breaker belt assembly with a further bandage ply of cords arranged parallel to the circumferential direction of the tire. There have been many proposals for multiple edge windings and/or different materials at the edge of such bandage pliers to provide extra reinforcement over the region of the belt edge. However, all of these proposals involve the use of extra material or an extra manufacturing step which add to the difficulty and expense of manufacture.
Accordingly there is a clear need in such elastomeric articles for a reinforcing ply which can provide different extensibility and thus different reinforcement in different parts within a single ply such that the ply may accommodate changes in shape without generating extreme differential stresses.